The present invention relates to composition friction elements and pertains particularly to improved friction units and method and composition for making same.
A friction brake is basically a pair of friction elements, one rotating and one stationary, brought into engagement to produce a friction force measured as brake torque for either slowing or stopping the rotating element. Brakes are designed so that the brake torque is somewhat proportional to the input force used to engage the elements. Unfortunately, pressure is not the only factor that influences the frictional response of the brake elements. Friction effects between friction elements cause friction force and brake torque to vary with engaging pressure, speed, and temperature, and to depend upon deposited interfacial film for stability.
The rotating element of a brake system is usually a steel disc or drum, and the stationary element is usually a composition pad or shoe lining. The materials forming the composition element are the principle unpredictable variables that have the greatest affect on the performance characteristics of the brake system. Desirable materials for the composition element must have good friction, wear and heat resistant characteristics. This includes good face resistance, or the ability to maintain good (preferably substantially uniform) braking with heat buildup.
Until recent years, the predominant material used in the manufacture of friction pads and discs for brakes, clutches and the like was asbestos. These were manufactured by a molding process where each unit was formed in a mold cavity. However, it was discovered that asbestos is a carcinogenic substance, and that such use released potentially harmful amounts of it into the environment. For this reason, some industrialized countries prohibit the use of asbestos friction materials, and others including the United States require the use of asbestos to be phased out over the next few years. Therefore, there exists an urgent need for safe and effective friction materials and economical methods of manufacturing the materials into suitable friction units.
Extensive efforts have been put forth in recent years in an effort to find suitable environmentally safe materials and compositions having the desirable wear, heat and other characteristics to serve as a substitute for asbestos. These efforts have been frustrated by the many and varied parameters involved, including the range of needs to be met. For example, different size vehicles require different size friction pads and often have other variables including higher operating forces and temperatures.
Attempts to satisfy the need for long life, high friction heat resistant friction materials have included proposals to utilize various chopped fibers molded in a bonding matrix, such as a resin. The friction unit is formed in the traditional fashion by a molding process, with the fibers randomly oriented and placed in a binder, such as either a dry powder resin cured under heat and pressure, or placed in a liquid resin in a mold and cured. Examples of these compositions and manufacturing methods are disclosed in U.S. Pat. No. 4,119,591, granted Oct. 10, 1978 to Aldrich, U.S. Pat. No. 4,259,397, granted Mar. 31, 1981 to Saito et al., and U.S. Pat. No. 4,432,922, granted Feb. 21, 1984 to Kaufman et al.
However, friction units made by this method are expensive to manufacture and have not been satisfactory, because of their lack of uniformity in performance and durability. For example, units from the same batch may vary as much as 35% in performance characteristics. The non-uniformity of results has been found to be caused largely by a non-uniformity of distribution and orientation of the fibers in the matrix. This not only creates expensive inspection and quality control problems, it can also create maintenance problems, and sometimes even hazardous conditions. For example, pads that have been matched for performance at initial installation may vary over their useful life.
In my aforementioned patent applications, I disclosed improved compositions and methods of manufacture for brake pads and linings. However, continuous work on perfecting these indicate that further improvements in both compositions and methods of manufacture are desirable and have been developed as set forth herein. For example, insufficient transverse mechanical properties were found to be a problem among many samples produced by the pultrusion process. Another problem included excessive voids in some samples.
Accordingly, it is desirable that improved compositions, structures and methods be available to overcome the above and other problems of the prior art.